A “slant rhyme” is something that’s almost, but not quite, a perfect rhyme. Like “splat” is a rhyme for “flat” and a slant rhyme for “clap.”
The trick to a slant rhyme is that the vowel sounds normally stay the same, while consonants can be replaced with similar noises. An unvoiced lingual stop at the end of a rod (like “T”) can be replaced with, say, a labial stop (“P”) and the actual sound is going to be a lot like a rhyme, even if it doesn’t meet all the requirements. There’s even a Slant Rhyme Dictionary!
Slant rhymes are often used in rhymes that are intended for performance, both because they make it easier to find rhymes and because they combine the “words sound alike” aspect of rhymes with the “every word doesn’t sound exactly the same” aspect of things that aren’t totally boring.
Now I shall provide something that may appear to be irrelevant. I shall ask you to be patient while I introduce this seeming non sequitur, because I promise that I shall concluding with a thrilling synthesis of my two themes.
I was recently alerted via Facebook to Eminem’s claim that he can rhyme “orange.” Here’s what Mr. Inem has to say:
“People say that the word ‘orange’ doesn’t rhyme with anything, and that kind of pisses me off, because I can think of a lot of things that rhyme with orange […] If you enunciate it and make it more than one syllable — or-ange — you could say, like, ‘I put my or-ange four-inch door hinge in stor-age and ate por-ridge with Ge-orge.'”
At this point, you might think that I’m going to defend those as being slant-rhymes. I am not; I’m actually going to explain why those are actual, legitimate rhymes.
See, the concept of “rhyming” is based on what words actually sound like, not what they’re supposed to sound like. Depending on one’s accent, “pen” and “tin” might rhyme, or might not. Consider this couplet from the “Fat Albert” theme song:
You’ll have some fun now with me and all the gang,
Learnin’ from each other while we do our thing
That doesn’t rhyme. Clearly. However, what if “thing” were pronounced “thang”? And don’t say “But that’s not how that word’s pronounced,” because the fact is that lots of people do pronounce it like that. Then it’s a perfect rhyme! I have to admit that the actual theme song does not use that pronunciation, but I’m pretty sure it was supposed to.
Now, here’s the thing. Eminem is not a poet; he’s a performer. His speech up there was written to be said out loud. And he’s perfectly capable of putting a little extra enunciating into “OR-inj” and then putting more voicing into the end of “Four-inch” so that the sounds line up perfectly. And that means they do rhyme.